The characteristics of the long-period ground motion with a period range from 2 to 20 s observed in the southwestern Kanto district, Japan, are discussed. Dispersive wave trains are found in seismograms observed during a moderate earthquake near the Izu-Oshima Island. The observed group velocities are in good agreement with the ones calculated for Love wave in subsurface structures derived from seismic refraction work in the area. It is interpreted that the Love wave radiated from the source is amplified, then it propagates in sediments of the plain. Normal mode summation technique and 2D finite difference method are combined to forward modeling of the Love-wave propagation in the sediments. The subsurface structure from the source to the margin of the plain is modeled by a multi-layered structure derived from the dispersion analysis. A two-dimensional model is constructed from the margin to sites on the plain, considering the result of the seismic surveys. The observed displacements in the first 60 s are well simulated. However, later arrivals cannot be explained at all. This is probably related to the simple assumption of the subsurface structure from the source to the margin of the Kanto Plain in the Sagami Bay.
We investigate the feasibility of using microtremor measurements in short and long period ranges for the prediction of frequency-dependent amplification effects due to local site geology in the Granada basin and Granada city (Southern Spain). Following a seismic reflection profile crossing the Granada basin, microtremor measurements were recorded at 12 different sites. Peaks centered at 0.7 s (1.5 Hz) and 3-4 s are observed in the microtremor spectra. The 0.7 s peak is interpreted in terms of microtremors and the second one in terms of ocean-generated microseisms. The former is present at all sites on Pliocene and Quaternary sedimens while the 3-4 s peak is present at all sites regardless of the surface geology, although with differences in the spectral amplitude. In order to investigate the possible influence of geologic parameters on long period motion, the maximum spectral amplitude value in the range 1 to 5 s was estimated at all sites in Granada basin. A good linear relation between the maximum spectral amplitude value and surface geology and depth to underground basement was found. However, for short period range (T<1 s), only the surface geology has any relation with the maximum spectral amplitude value. For Granada city, differences in the microtremor spectra were found to depend on the local geology. In the downtown area, situated on alluvial clay and sand from the Quaternary age, differences of up to 10 times with respect to the hill zones (located on compact conglomerates of Pliocene age) are observed. Results from coda-Q analysis from local earthquakes in the Granada basin show overestimations of Qc, for lapse times of 70 s, of up to 4 times at stations situated on sediment when compared to those obtained on bedrock for the 1.5 Hz frequency and to a lesser extent for 3 Hz. This overestimation is explained in terms of seismic-wave amplification in the Granada basin.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.